Ink jet recording which creates images using water-soluble inks has made marked strides recently. Ink jet recording is a type of printing that creates images and characters by ejecting small droplets of ink based on various principles to allow them to land on recording sheets such as paper. Ink jet recording is fast and quiet, facilitates multicolor printing, is versatile in terms of recordable patterns, and is free from the need of development or fixation. Thus, its use has become rapidly widespread in various applications to enable printing machines to print various figures including Chinese characters as well as color images. Further, ink jet recording can produce images which stand comparison with prints made by offset printing according to a printing plate technique or by a color photographic technique. Furthermore, ink jet recording has been widely used in the full color field because a small number of copies can be printed more inexpensively than by offset printing or a photographic technique.
As a result of further progress in the technology, ink jet recording has recently become used in industrial printing (hereinafter, referred to as “ink jet printing”). Because large numbers of copies are printed in the industrial printing field, printing speed is important due to the tradeoff between productivity and printing costs. A printing speed suitable for ink jet printing is achieved with a printing machine that is equipped with a line head in which ink-ejecting heads are fixed in the entirety of the width direction perpendicular to the paper transport direction (hereinafter, such a printing machine will be referred to as “ink jet printing machine”) (see, for example, Patent Literature 1). More recently, rotary ink jet printing machines having a printing speed of 15 m/min or more, a higher speed of 60 m/min or more, and a still higher speed in excess of 120 m/min have been developed. Rotary ink jet printing machines are also utilized in on-demand printing applications such as printing of addresses, printing of customer information, printing of numbers, and printing of bar codes. Because ink jet printing machines allow for handling of variable information, their use is particularly found in on-demand printing. In such applications, a preferred manner of industrial printing is to preliminarily print fixed information by offset printing and to print variable information by ink jet printing.
The types of printing paper used for ink jet printing machines are broadly categorized into so-called plain printing paper such as high-quality paper and PPC paper, and coated printing paper having a distinct ink receiving layer on a paper substrate.
Low-cost plain printing paper represents the overwhelming majority of paper used for industrial ink jet printing machines. Frequent applications of plain printing paper include invoices and account statements, and ad-papers and direct mails, as well as a combination thereof, namely, so-called transaction mail promotion. Because of the absence of an ink-receiving layer, however, such plain printing paper is poor in ink absorption properties compared to coated printing paper, resulting in the occurrence of uneven printing.
Inks used in ink jet printing machines are more frequently aqueous pigment inks replacing aqueous dye inks. The use of aqueous pigment inks encounters different problems from those in the case of aqueous dye inks.
The water resistance of images has been an important quality requirement in the use of aqueous dye inks. That is, the dye inks are required not to spread under highly humid conditions or in the event where the image is brought into contact with water for any reason.
On the other hand, the abrasion resistance of images is one of the quality requirements in the use of aqueous pigment inks. If the abrasion resistance of images is low, any friction on images after printing and drying causes the pigment inks to become detached and smear the printed images.
Uneven printing in printed sections is a problem encountered with pigment inks. Uneven printing is a phenomenon in which printing paper exhibits a nonuniform density of an ink fixed in the final printed image after the ink is dried to cause uneven ink absorption properties during high speed printing. Because inks used in ink jet printing have a low concentration of color material, uneven printing tends to be more marked than in offset printing. The presence of uneven printing deteriorates the commercial value of prints.
Some types of plain-like ink jet recording paper are coated slightly with silica to achieve a higher printing density (see, for example, Patent Literatures 2 and 3). However, such recording paper is poor in offset printability and does not reflect any consideration of abrasion resistance. An improvement in printing density can be expected with plain ink jet recording paper that is coated with PVA to control the Stöckigt sizing degree (see, for example, Patent Literature 4). However, such recording paper is poor in abrasion resistance. Further, plain ink jet recording paper coated with colloidal silica and PVA achieves improved abrasion resistance (see, for example, Patent Literature 5), but is unsatisfactory in terms of offset printability.